Magnetic bearings

ABSTRACT

A magnetic bearing ( 1, 2, 3 ) has two bearing members ( 5, 7 ) each of which carries a set of bearing elements ( 4, 6 ) and the bearing elements ( 4 ) of a said set carried by one member ( 5 ) interleaved with the bearing elements ( 6 ) of a said set carried by the other member ( 7 ) to define three or more substantially parallel interleaf gaps ( 11 ) between successive elements ( 4, 6, 4 , . . . ) so that bearing forces can be developed as a result of magnetic shear stresses acting across those gaps ( 11 ). The magnetic bearing achieves its bearing forces as the sum of force contributions from a number of parallel (or nearly-parallel) airgaps ( 11 ) and each of these individual airgap force contributions comes about as the integration of magnetic shear stress over the airgap area brought about by causing lines of magnetic flux to cross the airgap at an angle to the normal. A source ( 3 ) of magneto-motive force is arranged that at least one set of flux lines ( 12 ) comes to exist which crosses three or more of the interleaf gaps ( 11 ) and at least the majority of the bearing force is developed as a result of magnetic shear stresses acting across such interleaf gaps ( 11 ).

FIELD OF THE INVENTION

[0001] This invention relates to magnetic bearings, both passive andactive magnetic bearings, and particularly, but not exclusively, tocompact, high-stiffness and high load capacity magnetic bearings.

BACKGROUND TO THE INVENTION Magnetic Bearings: Purposes and Attributes

[0002] The purpose of a magnetic bearing is to provide a force betweentwo major bearing members without contact occurring. This force issubsequently referred to as the bearing force. Consistent with thenormal definition of any bearing, a magnetic bearing allows free motionin one or more senses whilst providing the capability for exertingbearing forces in at least one other sense. Most magnetic bearings areemployed in rotating machines to separate the rotor and the stator.Magnetic bearings have the advantages of very low energy loss rate(given proper design), no contact between parts giving potentially verylong life, and the ability to withstand relatively high temperatures.

[0003] Magnetic bearings may be active or passive. Active magneticbearings sense the relative position of the two major bearing members.They then adjust the electric currents in coils such that the net forcebetween the two major bearing members has the appropriate magnitude anddirection. Passive magnetic bearings usually involve magnetic fieldsfrom permanent magnets but they may alternatively be constructed usingcoils of conductor to provide the magnetomotive force (MMF). Theelectric currents flowing in these coils are not, however, a strongfunction of the relative position of the two bearing members. Passivemagnetic bearings often operate on the basis of repulsion of like poles.

[0004] In a simple view, active magnetic bearings may be arbitrarilystiff in the sense that the smallest amount of relative movement betweenthe two major bearing members can be made to cause a finite amount offorce. There are obviously limitations to this associated with theability to sense extremely small motions and the need for theclosed-loop control system to be stable. However, it is broadly acceptedthat active bearings are generally orders of magnitude more stiff thantheir passive equivalents. Stiffness of a bearing is extremely importantfor acceptable dynamic properties and in order that the relativeposition of the two major bearing members is insensitive to theexternally-applied load existing between them.

[0005] Another extremely important attribute of any bearing isreliability. Active magnetic bearings are complicated systems involvingsensing, control, and power-currents. As such, there are very manypossible modes of failure other than straightforward mechanicalbreakage. By contrast, passive bearings tend to be extremely robust andreliable with very few modes of possible failure other than mechanicalbreakage.

[0006] A key attribute of any magnetic bearing is size. A second relatedand equally important attribute is total weight. It is generallyaccepted that for a given force rating, a radial magnetic bearing ismany times larger than its rolling-element counterpart.

Elementary Bearing Regions and the Central Surface

[0007] Consider bearings which comprise two major bearing membersbetween which some free relative motion is to be provided. In manycases, at least one of the free relative motions is a rotation. Therotation of any physical body of a scale above atomic scale involvestranslation of the particles at the surface of that body. To provide abearing which can offer forces to resist relative motions of the twomajor bearing members in some senses and yet allow free rotation aboutsome axis, it is both necessary and sufficient to provide regions withinthe bearing where translation is opposed along at least one axis andfree along at least one other axis. Such regions are referred to aselementary bearing regions.

[0008] The conventional ball bearing is useful for illustration. Thisbearing contains a finite number of elementary bearing regions—one perball—where relative translation of the inner race and the outer race isstiffly resisted along one direction and where translation of the innerrace and outer race is free in the other two directions. The stiffdirection for each individual ball at a given instant is along the(ball) diameter between contacts. This simple conceptual model obviouslyignores friction and viscous shear forces at the contacts. FIG. 1illustrates the elementary bearing region of a ball bearing.

[0009] The collective action of all of these elementary bearing regionsin the case of a ball bearing results in a bearing which provides freerotation about one axis but reacts against all net translations betweenthe two major bearing members and (for an angular contact ball bearing)against the rotations about the other two orthogonal axes of rotation.

[0010] A similar view can be taken of a cylindrical roller bearing. Eachroller can produce very stiff opposition to relative translations of theinner and outer race in one direction. It will allow very free movementin the direction of rolling. It provides some resistance to relativemovement of the inner and outer races in the axial direction althoughthis resistance is not usually used. For cylindrical roller bearings, wecan consider that there is one elementary bearing region for eachindividual roller. For roller bearings having conical rollers, considereach roller to comprise a large number of disc-like slices and theelementary bearing regions are revealed. FIG. 2 illustrates anelementary bearing region from a conical roller bearing.

[0011] It is straightforward to extend this view of all bearings whichaccommodate rotation to hydrostatic and hydrodynamic bearings. In thecase of hydrostatic bearings, the elementary bearing regions can beregarded as the individual locations where pressurised fluid is fed intothe cavity between the two major bearing members. FIG. 3 depicts anelementary bearing region from a hydrostatic bearing, showing,superimposed, a pressure distribution over such a location. In the caseof hydrodynamic bearings, the lubricant interlayer between the two majorbearing members can be decomposed into patches each of which exerts someforce to maintain a distance between the two major bearing members. FIG.4 illustrates one such patch, and the direction of relative motionbetween the bearing members.

[0012] Following the above logic, all bearings can be decomposed intosets of elementary bearing regions having at least one direction ofcomparatively free relative translation and at least one direction inwhich translation is (or can be) strongly opposed.

[0013] In each of the above examples of bearings, the elementary bearingregions include a portion of the surface of each of the two majorbearing members. Between these two surfaces, there is a central surface.This is any smooth surface such that the action of the bearing region inproviding a direction of free translation can be considered to beequivalent to sliding of one side of this central surface relative tothe other. The term central surface is used regularly throughout theremainder of this document.

[0014] In most cases, the elementary bearing regions are only (orpredominantly) used to provide free translation in one direction. Thisdirection is in the plane of the central surface. Thus, it is possibleto establish an axis set of principal directions for an elementarybearing region according to FIG. 5 in which the three axes are:

[0015] (1) The axis of (predominant) free relative translation. Forobvious practical reasons, the free relative translation is similar to adiscrete pure shearing action at the central surface. This direction isarbitrarily labelled x in FIG. 5.

[0016] (2) The axis normal to the central surface. This direction isarbitrarily labelled z in FIG. 5.

[0017] (3) The remaining orthogonal axis, labelled direction y in FIG.5.

[0018] In all of the above cases, the force acting between the twosurfaces of the two major bearing members is predominantly along thenormal to the central surface i.e. along the z direction of FIG. 5.

[0019] No practical bearing at scales above atomic scales is trulylossless. There is some rolling resistance in ball and roller bearings.There is some viscous drag in the bearing fluids in hydrostatic andhydrodynamic bearings. There are eddy-current losses and hysteresislosses in magnetic bearings. Thus, in all cases, there is invariablysome component of force acting to oppose the relative translation of thetwo surfaces in the “free” direction, x.

Magnetic Stresses in Magnetic Bearings

[0020] Many existing designs of magnetic bearings rely squarely on thefact that where magnetic flux is caused to pass through air, there iseffectively a tensile Maxwell stress in the air in the direction of thelines of magnetic flux. Most, if not all, active magnetic bearingscurrently available operate directly on the basis of this tensilestress.

[0021]FIG. 6 illustrates the action of the tensile Maxwell stress inprobably the simplest instance where a horse-shoe shaped permanentmagnet drives a magnetic field through itself, an airgap (twice) andsome second body. Because the lines of magnetic flux in this case arepredominantly normal to the faces of the horse-shoe magnet and to thesurface of the second body, it is possible to approximate the net forcegenerated at each of the two airgap-crossings by a simple formula. Thesetwo discrete forces can then be combined using elementary trigonometryto produce an expression for the total resultant attractive forcebetween the magnet and the second body.

[0022] The oldest designs of active magnetic bearing compriseseparately-energised horse-shoe shaped electromagnets arranged about thecircumference of an airgap with a solid (or hollow) cylindrical rotor inthe centre. Each horse-shoe electromagnet has its own complete magneticcircuit and there is very little interaction between distinctelectromagnets. In normal operation, each electromagnet has a bias fieldsuch that there is always some magnetic flux through the horse-shoeelectromagnet. The bias field is sometimes provided by a DC component ofcurrent in the electromagnet but it can be provided by a permanentmagnet in the magnetic circuit. The forces produced by the bias fieldsgenerally sum to near zero. Then by introducing a relatively smallamount of (additional) current in one horse-shoe electromagnet and thenegative of this (additional) current in the horse-shoe electromagnetdiametrically opposite, a net transverse force is created between thebearing stator and the bearing rotor.

[0023] Some more modern designs of magnetic bearing utilise statorshapes which are akin to the stators of switched-reluctance machines inthat there are inwardly-protruding stator poles mounted onto acontinuous cylinder of back-iron. There may be coils on individualstator poles or coils may link two or more poles. Alternatively coilsmay be formed around the back of core following the old Gram-ringwinding method which was common in electrical machines some years ago.Permanent magnets may be provided in the stator poles or in the cylinderof back-iron to create the bias field. The relationship betweenindividual currents in coils (or phases) and the quantity of magneticflux passing through the individual stator poles is more complex inthese cases than it is for the simple arrangement of multipleindependent horse-shoe electromagnets. However the basic principle ofoperation is the same: attractive force per pole is (roughly)proportional to the square of total flux through the pole-face.

[0024] Most magneto-mechanical devices are fundamentally limited by fluxdensity. It is very rare for flux densities in any iron-containingmachine to rise above 2 Tesla anywhere in the iron because ofsaturation. (The word iron is used here to encompass any ferromagneticmaterial). Maximum flux density in a ferro-magnetic material is a keyparameter in choosing such a material for an application but it is notthe only one. Mechanical strength, stiffness, resistivity (foreddy-current losses) and low magnetic hysteresis effects are otherproperties that the designer must keep in mind when selecting a materialfor use in a magneto-mechanical device. Of course, there is ultimatelyno maximum magnetic flux density in iron or any other material but the(incremental) relative permeability for iron can fall from over 1000 atlow flux levels to not much above 1 at flux levels over 2 Tesla.

[0025] Magnetic flux densities in the iron of an iron-carryingmagneto-mechanical device are invariably higher than those in theairgaps where the magnetic flux is effective in generating force. Theterm airgap is used in this context to mean a region of space that mayor may not be filled by a non-magnetic fluid. This usage is consistentwith the interpretation of the term in the context of electricalmachines. Most usually, the gap between relatively movable parts of thedevice is occupied by air.

[0026] Given that airgap flux density is limited, it follows that theMaxwell stresses achievable are also limited in magnitude. The net forceor torque acting through an airgap can be computed by choosing anysurface through that airgap and integrating the magnetic stresses overthat surface. If this is done, an average effective airgap stress can bederived as the total force divided by the total airgap area or the totaltorque divided by the total first-moment of airgap area about the axisof rotation. The average airgap stress is limited to about 0.4 MPa.

[0027] In the context of the design of any magnetic bearing, a keyrequirement is to be able to develop a certain nominal force capable ofresisting motion in one direction. Given that the effective airgapstress in any magneto-mechanical device is inherently limited bysaturation of iron, it follows that there is a minimum operative area ofairgap for a given rated load. One route taken by designers of magneticbearings is to use relatively large flat bearing surface areas throughwhich magnetic flux passes. Another route taken is to use relativelylarge-diameter/long bearing surfaces so that the requisite airgap areacan be achieved in a finite length of shaft.

[0028] For a given magnetic flux density, B, in the airgap, the tensileMaxwell stress in the direction, “r”, of the lines of flux is given by:$\sigma_{rr} = \frac{B^{2}}{2\quad \mu_{0}}$

[0029] A fact that is much neglected in the design of magnetic bearingsis that in the two directions, “S” and “t”, perpendicular to r there iseffectively a compressive stress given by:$\sigma_{ss} = {{- \frac{B^{2}}{2\quad \mu_{0}}} = \sigma_{tt}}$

[0030]FIG. 7a shows a set of magnetic flux lines in a plane of constantt. The square box drawn in FIG. 7a can be considered to have tension,σ_(rr), acting on two opposite faces and compression (negative tension),σ_(ss), acting on the other two opposite faces. FIG. 7b shows the sameset of magnetic flux lines in the same plane of constant t. A square boxof the same size as that in FIG. 7a is drawn here also but theorientation of this square box is at 45° to the orientation of the boxin FIG. 7a. In this figure, axes “u” and “v” are defined to occur at 45°angles to the direction of the magnetic flux. On the sides of this box,it is found that effectively a pure shear stress is acting with nocomponent of normal stress. The magnitude of this pure shear stress“τ_(UV)” (in FIG. 7b) is identical to the magnitude of the normalstresses on the sides of the box in FIG. 7a.$\tau_{ss} = \frac{B^{2}}{2\quad \mu_{0}}$

[0031] Returning to the discussion of elementary bearing regions,consider that lines of magnetic flux are passing between the twobounding surfaces of the elementary bearing region in FIG. 5 such thateach flux line is (at least approximately) perpendicular to the xdirection (the direction in which free relative motion of the twobounding surfaces is desired). Provided that this condition issatisfied, there will be component of force between the two majorcomponents in the x direction. If these lines of flux are parallel tothe z direction (normal to the central surface), then the force betweenthe two bounding surfaces will equal to the stress times the area, i.e.B²A/2μ₀ where B is the flux density and A is the area of the centralsurface.

[0032] If, as indicated in FIG. 8, the flux lines are all perpendicularto x and they lie at an angle α to the normal, Z, then there will becomponents of force between the two bounding surfaces of the elementarybearing region in directions y and z, given by:${F_{y} = {{\frac{B^{2}A}{2\quad \mu_{0}}{\sin \left( {2\quad \alpha} \right)}\quad F_{z}} = {\frac{B^{2}A}{2\quad \mu_{0}}{\cos \left( {2\quad \alpha} \right)}}}}\quad$

[0033] In FIG. 8, positive F_(y) acts to pull the upper bounding surfacein the −y direction and it acts to pull the lower bounding surface inthe +y direction. Positive F_(z) acts to pull the upper bounding surfacein the −z direction and it acts to pull the lower bounding surface inthe +z direction.

STATEMENT OF THE INVENTION

[0034] According to the present invention there is provided a magneticbearing wherein each of two bearing members carries a set of bearingelements and the bearing elements of a said set carried by one memberare interleaved with the bearing elements of a said set carried by theother member to define three or more substantially parallel interleafgaps between successive elements so that bearing forces can be developedas a result of magnetic shear stresses acting across those gaps.

[0035] A distinguishing feature of the most preferred embodiments of thepresent invention is that the magnetic bearing described achieves itsbearing forces as the sum of force contributions from a number ofparallel (or nearly-parallel) airgaps and each of these individualairgap force contributions comes about as the integration of magneticshear stress over the airgap area brought about by causing lines ofmagnetic flux to cross the airgap at an angle to the normal. Asubstantial proportion of the lines of magnetic flux present within thebearing at any time are effective in producing useful airgap shearstress at three or more parallel airgaps.

[0036] The invention may, for example, provide a magnetic bearingcomprising first and second bearing members each provided with at leasttwo projecting elements which are interleaved to define at least threegaps between successive elements of the two bearing members, and asource of magnetomotive force (MMF) such that lines of magnetic fluxcross interleaf gaps at an angle to the normal in order to generate amagnetic shear stress across each such gap thereby generating a bearingforce or forces between the bearing members.

[0037] The present invention provides a bearing which may be of robustconstruction and it enables passive magnetic bearings of high stiffnessto be obtained.

[0038] Advantageously, at least one source of magneto-motive force is inplace such that at least one set of flux lines comes to exist whichcrosses three or more of the interleaf gaps and wherein at least themajority of the bearing force is developed as a result of magnetic shearstresses acting across such interleaf gaps.

[0039] In preferred embodiments of the invention, the source(s) ofmagneto-motive force is or are arranged so that a single set of fluxlines comes to exist which crosses a set of three or more of theinterleaf gaps and wherein at least the majority of the bearing force isdeveloped as a result of magnetic shear stresses acting across that setof interleaf gaps. In order to create this effect, the magnetic flux iscaused to follow a zig-zag pattern as it passes through the interleavedstack of bearing elements. It is especially preferred that substantiallyall the interleaf gaps are contained within said set. FIG. 9 illustratesthis zig-zag path schematically.

[0040] A significant saving in weight and materials is possible as aresult of this arrangement and accordingly it promotes low cost and highspecific load capacity. A significant factor determining the weight ofany magnetic bearing is the material which is required to complete themagnetic flux circuit, that is, to conduct the flux from one side of theset of gaps where it is useful in creating some bearing forcecontribution, to the other. Arranging for all (or many of) the gaps tobe crossed by a single set of flux lines can minimise the weightassociated with the magnetic return path for a given maximum bearingforce capability.

[0041] Magnetic bearings in accordance with the invention may achieve ahigh load capacity through causing a reasonable working shear stress toexist at each one of numerous (nearly) parallel airgaps. To cause amagnetic shear stress to exist in an airgap, it is necessary to providea source of magneto-motive force providing a magnetic flux, and to causethat magnetic flux to cross the airgap at an angle. The highest shearforce for a given flux density will occur when that angle is 45°. It isthus advantageous to be able to control the way in which the lines ofmagnetic flux cross the various interleaf gaps of the bearing. Manydifferent configurations can be devised which can cause some shearstress to exist. However, there are essentially only three distinct waysin which the path of magnetic flux can be altered from the path that itwould naturally take through free space. These are (a) by placing someferromagnetic material in the flux path, (b) by placing some permanentmagnet material in the flux path, or (c) by placing some electriccurrent in the flux path.

[0042] In some preferred embodiments of the invention, electricallyconductive material is arranged within one or more of the interleavedbearing elements to allow the flow of electric currents in order toinfluence the path of magnetic flux across at least one interleaf gap.Alternatively, or in addition, permanent magnet material may bedistributed within the interleaved bearing elements in order toinfluence the path of magnetic flux across at least one interleaf gap.

[0043] In yet other preferred embodiments of the invention, materials ofdifferent magnetic permeabilities are distributed within the interleavedbearing elements in order to influence the path of magnetic flux acrossat least one interleaf gap. In such cases, ferro-magnetic material issuitably distributed within the interleaved bearing elements for thispurpose. Thus, ferro-magnetic material may be distributed pattern-wisewithin at least one of the interleaved bearing elements such that thereluctance experienced by a line of magnetic flux passing from one sideof the bearing element(s) to the other is a strong function of thelocation of that flux line; this dependence of reluctance on locationthen serving to influence the path of magnetic flux across at least oneinterleaf gap.

[0044] Any or all of these methods of influencing magnetic flux acrossat least one interleaf gap may be incorporated in a single embodiment ofthe invention. FIGS. 10, 11 and 12 indicate schematically how the threedifferent effects (non-uniform permeability in a bearing element,permanent magnet material in a bearing element and electric current inthe plane of a bearing element) can influence the path of magnetic fluxacross airgaps.

[0045] In those embodiments which contain some permanent magnet materialin one or more of the bearing elements or some distributions of electriccurrent in one or more of the bearing elements, it may or may not benecessary to provide a separate source of magneto-motive force.

[0046] In some preferred embodiments of the invention, the bearing isconstituted as an active bearing. In other preferred embodiments of theinvention, the bearing is constituted as a passive bearing. In yet otherpreferred embodiments of the invention, the source of magneto-motiveforce comprises a single coil for selectively adjusting the total fluxlinkage in which case the bearing is constituted as a semi-activebearing.

[0047] In some preferred embodiments of the invention, the bearing isconstituted as a linear bearing. In other preferred embodiments of theinvention, the bearing of the invention is constituted as a rotationalbearing. When constituted as a rotational bearing, the bearing may bearranged to create radial bearing forces or to create axial bearingforces.

[0048] Preferably one said bearing member has one more interleavingelement than the other. This promotes bearing symmetry, and it alsoentails that there are at least four such gaps. A shear stress isachieved in each of these gaps. By providing multiple gaps, the totalsurface area acting to generate the force or forces between the bearingmembers will be increased, and a high load capacity bearing can berealised. The width of the gaps present in the bearing has a minimumvalue determined by the achievable registration of the two bearingmembers.

[0049] The number of interleaving elements may be increased asconvenient to increase the number of interleaf gaps and thus the totalsurface area acting to generate the force or forces between the bearingmembers. There are preferably at least six, and more preferably at leasteight or ten such interleaf gaps, and there may be as many as twenty-sixor even more.

[0050] The gaps between interleaving elements are optionally filled by anon-magnetic fluid. The gaps may be occupied by air. The gaps may beevacuated.

[0051] In those embodiments which contain non-uniform distributions offerro-magnetic material, the regions of high permeability may beconstituted by one or more ferromagnetic materials and the regions oflow permeability may be constituted by any non-ferromagnetic material,for example a composite such as a fibre-reinforced resin material.Carbon-fibre composites are particularly suitable.

[0052] The force or forces between the bearing members may be generatedin either one or two directions. The force or forces between the bearingmembers is preferably in a direction parallel to the central surfaces ofthe gaps. The force or forces between the bearing members act in adirection broadly parallel to the central surface of the gaps.

[0053] The elements of the first bearing member may be attachedtogether. The elements of the second bearing member may be attachedtogether. The elements may be mounted on a mechanical platform. Themechanical platform may be a shaft, or may be a sleeve that may befitted onto a shaft. The mechanical platform may be a shell which mayhold the elements together at their outer edges. The mechanical platformis preferably made from a non-magnetic material in order to prevent itfrom providing a magnetic short circuit.

[0054] The interleaving elements of the first and second bearing membersmay be annular discs. In a rotational bearing, such discs will bemounted normal to the axis of rotation. Such interleaving discs may bearranged to produce bearing forces which act normal to the bearingrotation axis.

[0055] The interleaving elements of the first and second bearing membersmay be cylinders. In a rotational bearing, such cylinders will bemounted coaxially with the axis of bearing rotation. Such interleavingcylinders may be arranged to produce bearing forces which act coaxiallywith or normal to the bearing rotation axis, according to the way inwhich the magnetic flux is caused to weave between the interleavingcylinders.

[0056] In yet further embodiments of the invention, the interleavingelements of the first and second bearing members may be conical. In arotational bearing, such conical elements will be mounted coaxially withthe axis of bearing rotation. Such interleaving conical elements may bearranged to produce bearing forces which act coaxially with or normal tothe bearing rotation axis, according to the way in which the magneticflux is caused to weave between the interleaving conical elements.

[0057] The interleaving elements, whether they be annular discs, orcylindrical or conical elements, or linear bearing elements, may beconstructed of laminated steel. Other construction methods or materialsare also possible including use of a powder metallurgy composites havinghigh resistivity and the use of composite material which comprise afraction of magnetic wire embedded in a matrix and appropriatelyoriented.

[0058] The MMF source or sources may comprise a series of permanentmagnets, or two concentric coils, or four identical pieces reminiscentin geometry of a G-clamp. The MMF source or sources may also serve tocomplete the magnetic path through the bearing. The MMF source orsources may comprise a return path on only one end of the bearing.

[0059] The first bearing member may be a rotor and the second bearingmember may be a stator of a rotating machine. Alternatively, the firstbearing member may be the stator and the second bearing member may bethe rotor of a rotating machine. The MMF source or sources arepreferably in the same frame of movement as the bearing stator whenprovided.

[0060] The magnetic bearing may be a passive magnetic bearing, or may bean active magnetic bearing. The magnetic bearing may be a compact andlight high-force capacity active magnetic bearing. The magnetic bearingmay be a compact high-stiffness passive magnetic bearing. This inventionprovides for a passive bearing having stiffness-per-unit-volume which ismany times larger than the stiffness-per-unit-volume offered byconventional designs of passive bearing. The magnetic bearing may be acompact high-stiffness passive magnetic bearing which permits relativerotation but resist relative axial motion by producing a restoringthrust. The magnetic bearing may be a passive thrust bearing havingaxial thrust capacity per unit volume/mass which is many times largerthan the thrust per unit volume offered by other designs of passivemagnetic thrust bearing. The magnetic bearing may provide for asubstantially higher axial stiffness per unit volume/mass which is muchhigher than that offered by other designs of passive magnetic thrustbearings. The magnetic bearing may provide for substantial axial thrustgiven a comparatively small amount of relative axial motion whileoffering little or no resistance to relative rotation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Some preferred embodiments of the present invention will now bedescribed by way of example only, with reference to the followingdiagrammatic drawings , in which:

[0062]FIG. 1 illustrates an elementary bearing region from a ballbearing;

[0063]FIG. 2 illustrates an elementary bearing region from a conicalroller bearing;

[0064]FIG. 3 illustrates an elementary bearing region from a hydrostaticbearing;

[0065]FIG. 4 illustrates a fluid wedge and relative motion in anelementary bearing region from a hydrodynamic bearing;

[0066]FIG. 5 illustrates the principal (orthogonal) directions for everyelemental bearing surface region, axis z being normal to the centralsurface;

[0067]FIG. 6 illustrates the action of tensile Maxwell stress in thesimplest where a horse-shoe shaped permanent magnet drives a magneticfield through itself, an airgap (twice) and some second body;

[0068]FIG. 7a shows normal stresses in a given plane of a magnetic fieldin air;

[0069]FIG. 7b shows shear stresses in a given plane of a magnetic fieldin air;

[0070]FIG. 8 illustrates magnetic flux passing at an angle, α, to the zaxis;

[0071]FIG. 9 illustrates magnetic flux passing in “zig-zag” patternthrough parallel airgaps;

[0072]FIG. 10 illustrates a bundle of magnetic flux being directed by aregion of high permeability;

[0073]FIG. 11 illustrates a bundle of magnetic flux being directed by apermanent magnet;

[0074]FIG. 12 illustrates a bundle of magnetic flux being directed by aline of current;

[0075]FIG. 13 is a cross sectional view of half of a passive radialbearing according to a first embodiment of the present invention;

[0076]FIG. 14 is a cross sectional view of half of the bearing rotor ofFIG. 13;

[0077]FIG. 15 is a cross sectional view of half of the bearing stator ofFIG. 13;

[0078]FIG. 16 is a plan view of a rotor plate bearing element of FIG.13;

[0079]FIG. 17 is a plan view of a stator plate bearing element of FIG.13;

[0080]FIG. 18 is a cross sectional view of the entire bearing of FIGS.13 to 17;

[0081]FIG. 19 is a plot of static restoring force as a function ofrelative lateral displacement between a bearing rotor and a bearingstator;

[0082]FIG. 20 is a schematic view of a first embodiment of the MMFsource of FIGS. 13 to 19;

[0083]FIG. 21 is a schematic view of a second embodiment of the MMFsource of FIGS. 13 to 19;

[0084]FIG. 22 is a schematic view of a third embodiment of the MMFsource of FIGS. 13 to 19;

[0085]FIG. 23 is a cross sectional view of half of a passive axialthrust bearing according to a second embodiment of the presentinvention;

[0086]FIG. 24 is a section through the bearing-rotor of the bearingshown in FIG. 23 in which regions (annuli) of high permeability areshown as dark rectangles;

[0087]FIG. 25 is a cross-section of the bearing-stator of the bearingshown in FIG. 23 in which regions (annuli) of high permeability areagain shown as dark rectangles;

[0088]FIG. 26 is a cross-section of the bearing-stator and bearing-rotorof the bearing shown in FIG. 23 with some axial displacement showing howreaction developed;

[0089]FIG. 27 is a schematic showing the 3 principal components and inparticular the coils (8) which contribute the homopolar MMF in thebearing shown in FIG. 23;

[0090]FIG. 28 is a cross-section parallel to the axis of rotation of aninterleaved-cylinders embodiment of active radial bearing according to athird embodiment of the invention in which lines of magnetic flux areshown as dashed vertical lines with arrows;

[0091]FIG. 29 is a cross-section normal to the axis of rotation of theembodiment of FIG. 28 in which some partial paths of magnetic flux shownas dashed zig-zag lines;

[0092]FIG. 30 is a cross-section normal to axis of rotation of thebearing-stator and MMF-sources of the embodiment of FIG. 28;

[0093]FIG. 31 is a cross-section parallel to axis of rotation of thebearing-stator of the embodiment of FIG. 28;

[0094]FIG. 32 is a cross-section normal to axis of rotation of thebearing-rotor of the embodiment of FIG. 28;

[0095]FIG. 33 is a cross-section parallel to axis of rotation of thebearing-rotor of the embodiment of FIG. 28;

[0096]FIG. 34 is an external MMF-source integral with the outermoststator-plate bearing element of the bearing illustrated in FIG. 28;

[0097]FIG. 35 is an internal MMF-source integral with the innermoststator-plate bearing element of the bearing illustrated in FIG. 28;

[0098]FIG. 36 is a cross-section parallel to the axis of rotation of aninterleaved-discs embodiment of active radial bearing according to afourth embodiment of the invention;

[0099]FIG. 37 is a front view of a single stator-plate bearing elementfrom the bearing of FIG. 36;

[0100]FIG. 38 is a section through the stator from the bearing of FIG.36;

[0101]FIG. 39 is a front view of a single rotor-plate bearing elementfrom -the bearing of FIG. 36;

[0102]FIG. 40 is a section through the rotor from the bearing of FIG.36;

[0103]FIG. 41 is a schematic showing the principal components of aninterleaved discs embodiment of active radial bearing according to afifth embodiment of the invention;

[0104]FIG. 42 is a schematic showing the principle of operation of thebearing of FIG. 41 illustrating how zig-zagging lines of flux create agood working shear stress at each inter-disc gap:

[0105]FIG. 43 are front and side views of an embodiment of MMF sourcefor use with the bearing of FIG. 41;

[0106]FIG. 44 is a front and side views of a single stator disc for usewith the bearing of FIG. 41;

[0107]FIG. 45 is a front and side views of a single rotor disc for usewith the bearing of FIG. 41 and showing its 4-pole axial magnetisation;

[0108]FIG. 46 is a sectional view through an embodiment of linearbearing constructed in accordance with the invention. This is the sixthembodiment described.

[0109]FIG. 47 is a sectional view through an embodiment of linearbearing constructed in accordance with the invention.

[0110] Reference has already been made to FIGS. 1 to 12.

[0111] Specific Embodiment “A”. A Passive Radial Magnetic Bearing.

[0112] FIGS. 13 to 18 show a passive radial magnetic bearing accordingto a first embodiment of the present invention. The magnetic bearingcomprises three main components, a bearing rotor member 1, a bearingstator member 2 and a magnetomotive force (MMF) source 3.

[0113] Recognising that the function of a so-called radial bearing is tomaintain a given relative lateral position of two bearing membersundergoing relative rotation, it is clear that either bearing member maystationary and the other one moving. Here and elsewhere in thedescription, the terms bearing rotor and bearing stator are used only todistinguish between the two bearing members. It will be implicitlyassumed, at least in the case of rotational bearings, that the MMFsource is in the same frame of movement as the bearing stator member.

[0114]FIG. 13 shows a cross section through half of the bearing rotor 1,the bearing stator 2 and the MMF source 3. In this figure, it is shownthat the MMF source also serves to complete the magnetic path. Thedashed lines 12 in FIG. 13 indicate the direction of flow of magneticflux. The MMF source provides a reasonably homopolar MMF through thebearing rotor and bearing stator. That is to say, any section throughthe bearing will reveal more or less the same pattern and density ofmagnetic flux passing through the rotor and stator.

[0115] The bearing rotor 1 comprises a number of circular rotor platebearing elements 4 mounted onto a single mechanical platform 5 as FIG.14 illustrates. The central planes of the rotor plate bearing elements 4are normal to the axis of rotation. The mechanical platform 5 is asleeve that may be fitted onto a shaft. The mechanical platform of thebearing rotor 1 is made from a non-magnetic material so that it does notprovide a magnetic short-circuit for the set of magnetic flux lines 12which pass through the rotor and stator plate bearing elements 4, 6, andthus also through the interleaf gaps 11 defined by those bearingelements.

[0116] The bearing stator 2 comprises a number of circular stator platebearing elements 6 mounted onto a single mechanical platform 7 as FIG.15 illustrates. Like the rotor plate bearing elements, the centralplanes of the stator plate bearing elements are also normal to the axisof rotation. The mechanical platform 7 is a shell which holds the statorplate bearing elements together at their outer diameters. The mechanicalplatform 7 of the bearing stator 2 is made from a non-magnetic materialso that it does not provide a magnetic short-circuit for the magneticflux intended to pass through the rotor and stator plate bearingelements.

[0117] All of the rotor plate bearing elements 4 are similar to eachother and all of the stator plate bearing elements 6 are also similar toeach other except that the two end-plate bearing elements may bedifferent in the sense that these may be integral with the MMF source 3.Any single rotor plate bearing element 4 appears almost identical to astator-plate bearing element 6. The principal difference is that theinnermost and outermost diameters on a rotor plate bearing element 4 areslightly smaller than those of a stator plate bearing element 6. FIG. 16shows a rotor plate bearing element 4 and FIG. 17 shows a stator platebearing element 6. Each of these plate bearing elements comprises a setof annular regions of high permeability 8 spaced apart by a set ofannular regions of relatively low permeability 9. The radial spacing ofthe regions of high permeability 8 is the same for the rotor platebearing elements as it is for the stator plate bearing elements and theregions of high permeability 8 have, to a very crude approximation, thesame radial depth as the regions of low permeability 9 on both thestator and rotor plate bearing elements. The dimensions of this radialspacing have been exaggerated in the diagrams for clarity. In practice,the radial pitch of the regions of high permeability 8 would be roughlyin the order of three times the maximum relative lateral movementallowable between the bearing rotor 1 and the bearing stator 2. Thedimension of the airgaps between the rotor and stator plate bearingelements would typically be around one half of this radial depth.

[0118]FIG. 18 shows a cross-section through the combined bearing rotorand bearing stator with a degree of lateral misalignment present betweenthem in the plane of the section. Only the rotor plate bearing elements4 and the stator plate bearing elements 6 are shown with the regions ofhigh permeability 8 and low permeability 9 in these plate bearingelements. Clearly the regions of high permeability 8 are not aligned.The effect of the MMF source is to try to drive magnetic flux axiallythrough the stack of rotor plate bearing elements 4 stator plate bearingelements 6 and airgaps therebetween. Where portions of the regions ofhigh permeability are aligned, relative high densities of flux arepassed but very little force is generated between the rotor and thestator where this occurs. Where portions of the regions of highpermeability are not aligned, the total reluctance of the axial magneticpath between the two ends is higher and such magnetic flux as does flowalong this path is forced to follow a “zig-zag” trajectory. FIG. 18includes lines 10 indicating this zig-zag trajectory broadly as magneticflux attempts to pass axially through the stack of interleaved rotorplate bearing elements 4 and stator plate bearing elements 6. The factthat the flux passes through each airgap at an angle means that there issome useful shear stress present and the effect of this shear stress isto try to pull the rotor and stator back into lateral (radial)alignment.

[0119]FIG. 19 shows a typical plot of static restoring force as afunction of relative lateral displacement between bearing rotor andbearing stator for a fixed axial MMF in the MMF source. Evidently, thereis some relative displacement, δ_(max), above which little additionalforce is available if displacement is increased further. Thisdeflection, δ_(max), is approximately equal to one quarter of the meanradial distance between the centres of adjacent regions of highpermeability on the rotor or stator plate bearing elements. The maximumanticipated relative deflection between the bearing rotor and bearingstator therefore provides a lowest bound for the radial spacing ofregions of high permeability in the rotor and stator plate bearingelements.

[0120] The thickness of airgaps present in the bearing has a minimumvalue determined by the achievable axial registration of the bearingrotor relative to the bearing stator. In order for the bearing to beeffective, the radial spacing between regions of high permeability mustbe substantially greater than the mean airgap thickness—typically 2 to10 times greater.

[0121] Hence achievable axial registration effectively places anotherlower bound on radial spacing of the regions of high permeability.

[0122] The axial thicknesses of the rotor and stator discs have twoseparate lower bounds: shear stresses (τ_(rθ)) in the discs and the factthat these thicknesses should be substantially larger than the airgapaxial thickness. The thicknesses of the rotor and stator discs may wellvary with respect to radius. For reasons of withstanding shear stresses,the rotor plate bearing elements may be particularly thick at theirinner diameters, progressively reducing in thickness, e.g. uniformly, totheir outer diameters. The thickness of stator plate bearing elementswould be determined largely by the requirements for some minimumthickness at the small radii and for maintaining a productive airgap atthe other radii. The stator plate bearing elements may vary in thicknessso as to provide parallel sided gaps with interleaved rotor platebearing elements.

[0123] The actual MMF present in the MMF source may take any one ofnumerous forms. Often it may comprise a series of permanent magnetsstacked up in the return path as shown in FIG. 20. FIG. 21 shows how twoconcentric coils may be used to very similar effect. FIG. 22 shows howthe MMF source might for a given bearing be comprised of four identicalpieces reminiscent in geometry of a G-clamp. FIG. 22 only depicts onehalf of one G-clamp.

[0124] The regions of high permeability 8 in either the rotor or statorplate bearing elements or even both, may themselves be permanent magnetmaterials. The regions of low permeability 9 in the rotor plate bearingelements 4 might often be created as carbon-fibre (or other fibre)composite in order to promote stability and mechanical integrity in eachrotor plate bearing element.

[0125] Specific Embodiment “B”. A Passive Axial Magnetic Bearing.

[0126] FIGS. 23 to 27 show a passive axial magnetic bearing according toa second embodiment of the present invention. The magnetic bearingdescribed here provides for substantial axial thrust given acomparatively small amount of relative axial motion while offeringlittle or no resistance to relative rotation. The bearing comprisesthree main components, a bearing rotor member 21, a bearing statormember 22 and an MMF source 23.

[0127]FIG. 23 shows a cross section through half of the bearing rotor,the bearing stator and the MMF source. In this figure, it is shown thatthe MMF source also serves to complete the magnetic path. The dashedlines in FIG. 23 indicate the direction of flow of magnetic flux. TheMMF source provides a reasonably homopolar MMF through the bearing rotor21 and bearing stator 22. That is to say, any section through thebearing including the axis of rotation in the plane of section willreveal more or less the same pattern and density of magnetic fluxpassing through the rotor and stator.

[0128] The bearing rotor 21 comprises a number of concentric cylindricalrotor plate bearing elements 24 mounted onto a single mechanicalplatform 25 as FIG. 24 illustrates. FIG. 24 is a cross section throughthe bearing rotor 21 where the plane of section includes the axis ofrotation. The mechanical platform 25 is generally disc-shaped and it ismade from a non-magnetic material so that it does not provide a magneticshort-circuit for the magnetic flux intended to pass through the rotorand stator plate bearing elements.

[0129] Each of the rotor plate bearing elements 24 comprises a set ofring-shaped regions 28 of high relative magnetic permeability spacedapart at regular intervals by a set of ring-shaped regions 29 of lowrelative permeability.

[0130] The bearing stator 22 comprises a number of concentriccylindrical stator plate bearing elements 26 mounted onto a singlemechanical platform 27 as FIG. 25 illustrates. FIG. 25 is a crosssection through half of the bearing stator 22 where the plane of sectionincludes the axis of rotation. The mechanical platform 27 shown in FIG.25 is generally disc-shaped and it is made from a non-magnetic materialso that it does not provide a magnetic short-circuit for the magneticflux intended to pass through the rotor and stator plate bearingelements.

[0131] Each of the stator plate bearing elements 26 comprises a set ofring-shaped regions 28 of high relative magnetic permeability spacedapart at regular intervals by a set of ring-shaped regions 29 of lowrelative permeability.

[0132]FIG. 26 shows a cross section through half of the combined bearingrotor 21 and bearing stator 22 with a degree of axial misalignmentpresent between them in the plane of the section. Only the rotor platebearing elements 24 and the stator plate bearing elements 26 are shown,with the regions of high permeability 28 and low permeability 29 inthese plate bearing elements. Clearly the regions of high permeabilityare not aligned. The effect of the MMF source is to try to drivemagnetic flux radially through the stack of stator plate bearingelements 24, rotor plate bearing elements 26 and airgaps therebetween.Where portions of the regions of high permeability are aligned, theinterleaved arrangement of rotor plate bearing elements 24 and statorplate bearing elements 26 offers a relatively low reluctance path formagnetic flux and very little net force is generated between the rotorand the stator. Where portions of the regions of high permeability arenot aligned, the total reluctance of the axial magnetic path between thetwo ends is higher and the magnetic flux is forced to follow a “zig-zag”trajectory. FIG. 26 includes lines 30 indicating this zig-zag trajectorybroadly as magnetic flux attempts to pass radially through the stack ofinterleaved rotor and stator plate bearing elements. The fact that theflux passes through each airgap at an angle means that there is someuseful shear stress present and the effect of this shear stress is totry to pull the rotor and stator back into axial alignment.

[0133] The thickness of airgaps present in the bearing has a minimumvalue determined by the achievable transverse registration of thebearing rotor 21 relative to the bearing stator 22. In order for thebearing to be effective, the axial spacing between regions of highpermeability must be substantially greater than the mean airgapthickness. Hence achievable transverse registration effectively places alower bound on axial spacing of the regions of high permeability. Usingmore closely-spaced regions of high permeability makes for ahigher-stiffness axial bearing but it does not greatly influence theamount of axial force available. If the axial spacing becomes too smallrelative to the airgaps, the available axial force is compromised.

[0134] The radial thicknesses of the rotor cylindrical plate bearingelements 24 and stator cylindrical plate bearing elements 26 have twoseparate lower bounds: shear stresses (τ_(zθ)) in the cylinders and thefact that these thicknesses should be substantially larger than theairgap radial thickness. The thicknesses of the stator plate bearingelements and rotor plate bearing elements may well vary with respect toaxial position. For reasons of withstanding shear stresses, the rotorplate bearing elements may be particularly thick at their roots—near tothe mechanical platform 25 for the bearing-rotor 21. The same is truefor the stator plate bearing elements 26 for the same reasons.

[0135] The actual MMF present in the MMF source 23 may take any one ofnumerous forms. Often it may comprise a series of permanent magnetsstacked up in the return path either oriented axially in the“cylindrical” part of the MMF source 23 or radially in the “disc-shaped”part of the MMF source 23. FIG. 27 shows how the MMF can be createdsatisfactorily by providing coils 31 wound in order to drive homopolarflux.

[0136] The regions of low permeability in the rotor might often becreated as carbon-fibre (or other continuous-fibre) composite in orderto enhance mechanical integrity in the rotor plate bearing elements 24and stator plate bearing elements 26—particularly with regard towithstanding high rotational speeds.

[0137] In FIG. 23, the MMF source was shown as a two-sided provisioninsofar as there are radial return-paths for the magnetic flux throughthe MMF source at both ends of the bearing. In many instances, there maybe a return path on only one end of the bearing. The cross-sections ofthe regions of high permeability in the rotor plate bearing elements 24and stator plate bearing elements 26 are shown as rectangles in thefigures. Depending on the desired shape of the force-deflection curve,these might be shaped differently. In FIGS. 24 and 25, the regions oflow permeability in the rotor plate bearing elements and stator platebearing elements appear to have identical dimensions to the regions ofhigh permeability. In general this will not necessarily be the case. Oneargument prevails in at least some cases for reducing the axial-lengthsof the regions of high permeability relative to the regions of lowpermeability in order to reduce the requirement for a very thick returnpath in the MMF source. In FIG. 26, the shaft is a part of the magneticcircuit. In general the shaft may or may not form a part of the circuit.If the shaft is non-magnetic, an inner sleeve must be provided tocomplete the magnetic circuit. This sleeve may be physically connectedto either the rotor or the stator. Clearly, in at least some cases, itis sensible to connect it to the shaft. The regions of high permeabilityin either the rotor or stator plate bearing elements or both, maythemselves be made from permanent magnet material magnetised with ahomopolar magnetisation in the radial direction. In such cases, it mayor may not be necessary for the component named the MMF source tocontribute any net MMF to the magnetic circuit in which case its rolewould be purely one of completing the magnetic circuit.

[0138] Specific Embodiment “C”. An Active Radial Magnetic Bearing.

[0139] FIGS. 28 to 35 show an active radial magnetic bearing accordingto a third embodiment of the present invention. The magnetic bearingcomprises four main components, a bearing rotor member 33, a bearingstator member 34, an external MMF source 35 and an internal MMF source36. In the present embodiment, the internal and external MMF sources arein the same frame of movement as the bearing stator.

[0140]FIG. 28 shows a cross section through the bearing rotor, thebearing stator and the external and internal MMF sources. This sectionis taken parallel to the axis of rotation. The dashed lines in FIG. 28indicate the direction of flow of magnetic flux. The MMF sourcesgenerate a 2-pole MMF pattern through the bearing. That is to say, givenany diametral line at angle θ, the net MMF across the bearing along anysuch diametral line varies according to cos(θ+φ(t)) where φ(t) is sometime-dependent phase-shift. In this figure, it is implicit that the MMFsources 35, 36 also serve to complete the magnetic path—that is, toconduct the magnetic flux in circumferential directions.

[0141]FIG. 29 shows a cross section—normal to the axis ofrotation—through the bearing rotor, the bearing stator and the externaland internal MMF sources 35, 36. A set of concentric annuli is evidentwith each annulus having alternating regions 37 of low relativepermeability and regions 38 of high relative permeability (ferromagneticmaterial/composite). With the exceptions of the innermost and outermostannuli, alternating annuli in FIG. 29 belong to the bearing stator andbearing rotor respectively.

[0142] The innermost annulus represents the section through the internalMMF source 36. The outermost annulus represents the section through theexternal MMF source 35. Note that the detail of windings in these MMFsources is omitted for clarity in the figure. The task of designing awinding for the MMF-sources to produce a 2-pole MMF is a perfectlystandard part of the design of rotating electrical machines andvirtually all of the options available from the electrical machinesindustry are applicable in the present circumstances.

[0143]FIG. 29 also indicates some of the paths 39 taken by magnetic fluxthrough the bearing at a given instant in time. The strong zig-zagpattern in this flux is immediately evident meaning that there will be asubstantial airgap shear-stress across each individual airgap acting (inthe present instance) to pull the bearing rotor down and to pull thebearing stator up. (The lines of magnetic flux effectively try tostraighten out to minimise the reluctance of the magnetic path).

[0144] Evidently from FIG. 29, there is a very high reluctancepreventing any significant passage of flux through the upper half of thebearing at this instant. The flux would have to cross several fullregions of low permeability to pass through this area. There are somepaths of moderately low reluctance through the bottom half of thebearing at this instant and some finite amount of flux will pass throughthis. The net amount of force between bearing rotor and bearing statorresulting from this flux in the lower half of the bearing at thisinstant will be small.

[0145]FIG. 30 provides a cross section through the bearing stator 34 andthe two MMF sources 35,36 normal to the axis of rotation. The radialalignment of the regions of high permeability 38 on all of the statorelements is shown. FIG. 31 provides a section through the bearing stator34 parallel to and including the axis of rotation. This shows clearlyhow the bearing stator comprises a number of stator cylinders 40 mountedonto a single mechanical platform 41. The mechanical platform 41 of thebearing stator 34 is made from a non-magnetic material so that it doesnot provide a magnetic short-circuit for the magnetic flux intended topass through the rotor and stator elements.

[0146]FIG. 32 provides a cross section through the bearing rotor 33normal to the axis of rotation. The radial alignment of the regions ofhigh permeability 38 on all of the rotor elements is shown. FIG. 33provides a section through the bearing stator 33 parallel to the axis ofrotation. This shows clearly that the bearing rotor 33 comprises anumber of rotor cylinders 42 mounted onto a single mechanical platform43. This mechanical platform 43 is also made from a non-magneticmaterial so that it does not provide a magnetic short-circuit for themagnetic flux intended to pass through the rotor and stator elements.

[0147] The number of regions of high permeability 38 on each statorcylinder 40 is the same and these regions are spaced out at even angularincrements. In general, this number is denoted N_(S). The number ofregions of high permeability 38 on each rotor cylinder 42 is the sameand these regions are also spaced out at even angular increments. Ingeneral, this number is denoted N_(R). The two numbers, N_(S) and N_(R)differ by 1. In the present case, N_(S)=20 and N_(R)=21.

[0148] At any given instant, there will be one direction where it ispossible to generate a substantial force between bearing stator 34 andbearing rotor 33. If this direction is being viewed from the frame ofthe stator, this direction rotates at a frequency of N_(R) times thespeed of relative rotation of stator and rotor. If this “direction” isbeing viewed from the frame of the rotor, this direction rotates at afrequency of N_(S) times the speed of relative rotation of stator androtor. Thus, for example, if the stator of the present embodiment isstationary and the rotor is spinning at 100 cycles per second, there are2100 individual opportunities in each second in which an impulse can beimparted to the rotor in any given direction. By varying the magnitudeand direction of the applied MMF field, very strong frequency componentsof force can be achieved in any direction for frequencies up to 1050 Hzin this case. In the general case, the frequency limit (before aliasing)is N_(R)Ω/2 where Ω is the shaft rotational speed.

[0149] Obviously, since this bearing operates by supplying a set ofimpulses in place of a steady force, there is some possibility thatrotor or stator resonances might be excited. Hence careful choice of thebearing location and bearing support properties is mandatory. Bycorrectly shaping the current waveforms in the MMF sources, the harmoniccontent of the net relative force above N_(R)Ω/2 can be reduced toarbitrarily low levels.

[0150] The thickness of airgaps present between rotor and statorcylinders in the bearing obviously have a minimum value determined bythe allowable transverse misalignment that the bearing is expected toaccommodate. Normally the smallest airgap would be many times greaterthan this minimum value. The airgaps between adjacent rotor and statorcylinders in this embodiment increase with increasingradius—approximately in proportion.

[0151]FIGS. 29, 30 and 31 imply that only 7 active airgaps may bepresent. In an actual implementation, the number of airgaps might besubstantially larger. Conceivably, in some cases there might be fewerairgaps. However in such cases, it is possible that a more conventionaldesign of bearing would have a higher force capability.

[0152] In the above description, the internal MMF source 36 is active inproviding some of the MMF to drive the magnetic flux through the rotorand stator cylinders and it is fixed in the same frame of motion as theexternal MMF source 35. The internal MMF source need not necessarilycontribute any net MMF to the magnetic field in which case, it can befree to rotate with the rotor. The internal MMF source may comprise asimple stack of annulus-shaped laminations in these cases—serving onlyto conduct magnetic flux across the central portion of the bearing.Alternatively, the external MMF source may not be required to contributeany net MMF if sufficient MMF can be provided by the internal MMF sourcein which case the external MMF source would be free to rotate with therotor. It could comprise a simple stack of annulus-shaped laminations inthese cases—serving only to conduct magnetic flux circumferentiallyaround the outside of the bearing.

[0153] The radial thicknesses of the rotor and stator cylinders 40, 42are shown as being constant in FIGS. 29, 30 and 31 but in some optimisedcases, these would vary along the axial length. Because bearing-force isaccumulated along the length, there may sometimes be a need for theroots of the cylinders on both the rotor and the stator (near themechanical platform in both cases) to be radially thicker than the tips.

[0154] When the external MMF source 35 is designed to contribute a netMMF to the magnetic circuit, it is sensible to make this componentintegral with the outermost stator cylinder so that the regions of lowpermeability of this stator cylinder are occupied by windings.Similarly, when the internal MMF source 36 is designed to contribute anet MMF to the magnetic circuit, it is sensible to make this componentintegral with the innermost stator cylinder so that the regions of lowpermeability of this stator cylinder are occupied by windings. FIGS. 34and 35 illustrate these circumstances.

[0155] Note that in FIG. 28, there is a rotor cylinder adjacent to theinternal MMF source 36 and not a stator cylinder. Obviously, when theinternal MMF source and the innermost stator cylinder are one integralunit, there would not be a rotor cylinder between them.

[0156] Specific Embodiment “D”. An Active Radial Magnetic Bearing.

[0157] FIGS. 36 to 40 show an active radial magnetic bearing accordingto a fourth embodiment of the present invention. The magnetic bearingcomprises four main components, a bearing rotor member 50, a bearingstator member 51 and two MMF sources 52.

[0158] In the present embodiment the net direction of magnetic fluxthrough the bearing is in the axial direction. This is in contrast tothe embodiment previously described where the net direction of magneticflux was orthogonal to the axis. In the embodiment described here, bothMMF sources are identical and they are in the same frame of movement asthe bearing stator.

[0159]FIG. 36 shows a cross section through the bearing rotor 50, thebearing stator 51 and the two MMF sources 52. This section is takenparallel to the axis of rotation. The lines in FIG. 36 indicate thedirection of flow of magnetic flux. The magnetic circuit is completedinternally within the MMF sources 52. The MMF sources generate a 2-poleMMF pattern through the bearing. That is to say, magnetic flux is pushedaxially along one side of the bearing and it returns along the otherside.

[0160] The bearing stator 51 comprises a set of stator plate bearingelements 53 (FIG. 38) whose central surfaces are parallel to each otherand normal to the axis of rotation. These stator plate bearing elements53 are disc-shaped, and comprise sectors of alternating highpermeability 54 and low relative permeability 55 as indicated in FIG.37. They are mechanically joined together by a common mechanicalplatform 56 (FIG. 38). The mechanical platform is made from anon-magnetic material to prevent it from short-circuiting the magneticcircuit. FIG. 37 shows a single stator plate bearing element havingN_(S) (=20 in this case) sectors of high relative permeability 54.

[0161] The bearing rotor 50 comprises a set of rotor plate bearingelements 57 (FIG. 40) whose central surfaces are parallel to each otherand normal to the axis of rotation. These rotor plate bearing elements57 are disc-shaped, and comprise sectors of alternating highpermeability 54 and low relative permeability 55 as indicated by FIG.39. They are mechanically joined together by a common mechanicalplatform 58 (FIG. 40), which may be a sleeve to fit over a shaft or itmay be the shaft of the rotor of the rotating machine itself. Themechanical platform is made from a non-magnetic material to prevent itfrom short-circuiting the magnetic circuit. FIG. 39 shows a single rotorplate bearing element having N_(R) (=21 in this case) sectors of highrelative permeability 54.

[0162] The principle of operation of this bearing is identical to thatof the bearing described in the third embodiment above. At any giveninstant, there will be one direction where it is possible to generate asubstantial force between stator and rotor. If this direction is beingviewed from the frame of the stator, this direction rotates at afrequency of N_(R) times the speed of relative rotation of stator androtor. If this direction is being viewed from the frame of the rotor,this direction rotates at a frequency of N_(S) times the speed ofrelative rotation of stator and rotor. Thus, for example, if the statorof the present embodiment is stationary and the rotor is spinning at 100cycles per second, there are 2100 individual opportunities in eachsecond in which an impulse can be imparted to the rotor in any givendirection. By varying the magnitude and direction of the applied MMFfield, very strong frequency components of force can be achieved in anydirection for frequencies up to 1050 Hz in this case. In the generalcase, the frequency limit (before aliasing) is N_(R)Ω/2 where Ω is theshaft rotational speed.

[0163] Obviously, since this bearing operates by supplying a set ofimpulses in place of a steady force, there is some possibility thatrotor or stator resonances might be excited. Hence careful choice of thebearing location and bearing support properties is mandatory. Bycorrectly shaping the current waveforms in the MMF sources, the harmoniccontent of the net relative force can be minimised.

[0164] The thickness of airgaps present between rotor and stator platebearing elements in this embodiment of the bearing obviously has aminimum value determined by the allowable axial misalignment that thebearing is expected to accommodate. Normally the smallest airgap wouldbe many times greater than this minimum value. The airgaps betweenadjacent rotor and stator plate bearing elements in this embodimentincrease with increasing radius—approximately in proportion. Thisincrease in airgap would be accommodated primarily by a correspondingdecrease in the axial thickness of the rotor plate bearing elements.There may also be some variation in the axial thickness of the statorplate bearing elements with radius.

[0165] Specific Embodiment “E”. An Active Radial Magnetic Bearing.

[0166] FIGS. 41 to 45 describe a magnetic bearing according to a fifthembodiment of the present invention. This magnetic bearing, like thosein the above embodiments, achieves a high load capacity through causinga reasonable working shear stress to exist at each one of numerous(nearly) parallel airgaps.

[0167] The bearing of this embodiment achieves the necessary obliquenessof flux within the airgaps by employing a distribution of electriccurrents in a layer on at least one side of each airgap. On the otherside of each airgap another layer is located in which there is eitheranother distribution of electric current or a distribution of permanentmagnet material. Whether there is a current distribution or adistribution of permanent magnet material in a given layer, the neteffect is nevertheless a provision of axial MMF in the layer whichvaries according to position within the layer.

[0168] This bearing is similar in geometry to the bearing described inthe fourth embodiment above. However, the means by which magnetic fluxis redirected here is quite different—being based on permanent magnetsand current distributions whereas in the fourth embodiment, theredirection of magnetic flux was based on regions of high ferromagneticpermeability.

[0169] The magnetic bearing of this embodiment comprises three maincomponents, a bearing rotor member 60, a bearing stator member 61 andtwo external MMF sources 62 (FIG. 41) which are again in the same frameof movement as the bearing stator.

[0170]FIG. 41 shows a cross section through the bearing rotor 60, thebearing stator 61 and the external MMF sources 62. This section is takenparallel to the axis of rotation. The net direction of flow of magneticflux is axial as indicated in FIG. 41 using arrows. The external MMFsources 62 generate a 2-pole MMF pattern through the bearing. That is tosay, given any diametral line at angle θ, the net MMF across the bearingalong any such diametral line at any instant is proportional to cos(θ+φ)for some phase angle, φ and it is independent of position along thatdiametral line. The external MMF sources also serve to complete themagnetic path—that is, to conduct the magnetic flux in circumferentialdirections at the two ends of the bearing.

[0171] The bearing rotor 60 comprises a number of rotor discs 63 in astack and the bearing stator 61 comprises a number of stator discs 64 ina stack (FIG. 42). The rotor discs 63 and stator discs 64 are both“layers” in the sense applied above and as such, they all have provisionfor axial MMF.

[0172]FIG. 42 indicates schematically how the bearing achieves a netlateral force. Lines of magnetic flux pass axially along the bearingcutting stator discs 64 and rotor discs 63 in alternation. Eachindividual stator disc 64 provides an axial MMF pattern varying(approximately) according to cos(θ+φ) where φ is a phase angle. Eachindividual rotor disc 63 provides an axial MMF pattern varying(approximately) according to cos(2θ+ψ) where ψ is a phase angle. In FIG.42, φ is set to zero and ψ is set to −45°.

[0173] A line of magnetic flux passing axially through the centre of astator disc 64 at θ=0° will naturally attempt to pass through (or closeto) the centre of the adjacent rotor disc 63 at θ=45° and it will returnto the line θ=0° when it passes again through the centre of a statordisc 64. Similarly, a line of magnetic flux passing through the centreof a stator disc 64 at θ=180° will naturally attempt to pass through (orclose to) the centre of the adjacent rotor disc 63 at θ=135° and it willreturn to the line θ=180° when it passes again through the centre of astator disc 64. By symmetry, there is no net axial flux in the planeθ=±90°. If the flux pattern is viewed from the side, it is seen that alllines of magnetic flux rise to enter a rotor disc 63 and fall as theleave the other side of the rotor disc 63. This behaviour provides therequisite angle in the magnetic flux to create a substantial mean shearstress tending to pull all rotor discs downwards relative to the statordiscs.

[0174] This embodiment uses distributions of permanent magnet materialfor the rotor discs 63 and distributions of radial current for thestator discs 64. Magnetic iron is employed to provide structuralrigidity and strength without substantially impeding the axial traverseof magnetic flux. The magnetic iron does not contribute significantly tothe redirection of magnetic flux in this case—unlike the previousembodiments.

[0175] The external MMF sources 62 each comprise a toothed disc 65 (FIG.43) and a set of windings. The toothed disc is of a laminatedconstruction comprising either a single coil of thin laminated magneticiron wound on flat or a set of concentric thin cylinders having a verythin insulating layer between adjacent cylinders. This constructionensures that alternating magnetic flux can pass through the toothed disc65 in axial and circumferential directions with minimal eddy-currentlosses. It does not matter that any magnetic flux attempting to pass ina radial direction within the toothed disc 65 incurs much moresignificant eddy-current losses as no such component of flux isrequired. FIG. 43 shows the toothed disc 65 in front and sideelevations.

[0176] A single MMF source coil 66 is shown in FIG. 43 linking a numberof the teeth. The number of conductors in this coil, and the thicknessof insulation on those conductors are both governed by the voltage dropand current load expected on this coil. The MMF source coils 66 areseries-connected into groups and these groups are connected in parallelinto phases following standard practice in the construction ofdisc-shaped electrical machines. The set of windings on each externalMMF source 62 comprises at least 2 independent phases such that arotating 2-pole axial magnetic field can be generated. That is to say,given any straight line parallel to the axis of the magnetic bearing andlocated at angle, θ, and radius, r, reaching between the two externalMMF sources the net MMF along that line contributed by the pair ofexternal MMF sources is determined approximately according to cos(θ+φ)and it is independent of radius. In this, the angle, φ, is a phase anglewhich can be controlled to be any value between 0 and 2π depending onthe values of the phase currents in the set of windings.

[0177]FIG. 44 shows a single stator disc 67. Each stator disc 67 carriesa set of stator disc coils 68 arranged into a stator disc winding. Thestator disc is relatively thin in the axial direction. Its constructionis such that it allows the passage of alternating magnetic flux in theaxial direction with minimal eddy-current losses. In the presentembodiment, its construction is from thin laminated magnetic iron woundon flat to produce a dense spiral. The stator disc 67 has teeth machinedin each side and the stator disc coils 68 are laid into these teeth withthe set of stator disc coils 68 on one side of the stator disc 67 beinga mirror image of the stator disc coils 68 on other side of the statordisc 67. The stator disc coils are series-connected into groups andthese groups are connected in parallel into phases in a patternidentical to that used for the windings formed from the MMF sourcecoils.

[0178] The phases of each stator disc 67 are electrically connected withthe phases of the external MMF sources 62 such that when these phasesare energised, the axial component of magnetic flux density isreasonably uniform with axial position along the magnetic bearing. Ifthe entire bearing rotor 60 was magnetically inert, the magnetic fluxwithin the bearing would be predominantly in an axial direction atalmost every position and its distribution would be approximatelyrepresented by cos(θ+φ) where the angle, φ, is again a phase angle whichcan be controlled to be any value between 0 and 2π depending on thevalues of the phase currents in the set of windings.

[0179]FIG. 45 shows a single rotor disc 69 and indicates the 4-polepattern of axial magnetisation. In the present embodiment, the rotordisc comprises a distribution of permanent magnet material magnetised inthe axial direction such that the net MMF contributed to any line ofmagnetic flux passing axially from one side of the rotor disc 69 to theother varies according to cos(2θ+ψ) where the angle, ψ, is some phaseangle controlled by the angle of rotation of the rotor. At somereference position of the rotor, ψ=0.

[0180] At any given angular position of the rotor, it is straightforwardto reason from FIG. 42 that it is possible to create forces in twoorthogonal directions by energising the MMF source coils 66 and thestator disc coils 68 in an appropriate manner.

[0181] Specific Embodiment “F”. A Passive Radial Magnetic Bearing.

[0182] A sixth embodiment will now be described, which is identical tothe fifth embodiment described above except that the requisite patternof axial MMFs in the rotor discs 63 is generated using a set of windingsin this case—in contrast to the previous embodiment in which permanentmagnet material was used.

[0183] The windings on the rotor discs 63 can be very similar in form tothose on the stator discs 64 except that the number of magnetic poles onthe rotor discs 63 must always differ from the number of poles on thestator discs (5) by ±2. As in the previous embodiment, the preferredpole numbers are 2 poles for the stator field and 4 poles for the rotorfield. One key difference between the windings on the rotor discs 63 andthose on the stator discs 64 is that the distribution of axial MMF onthe rotor discs need not be rotated relative to the rotor discs andhence there is not a need for two or more electrical phases on therotor. The windings on all of the rotor discs 63 are electricallyconnected together so that each rotor disc produces a similardistribution of axial MMF at all times.

[0184] Both of the fifth and sixth embodiments described abovepresuppose that the predominant direction of working magnetic fluxthrough the bearing will be axial and that the parallel airgaps willtherefore be disc-shaped airgaps lying between rotor discs 63 and statordiscs 64. In fact, it is conceptually simple to develop the same thoughtto a system in which the parallel airgaps are cylindrical—lying betweenparallel rotor and stator cylinders. The predominant direction ofmagnetic flux in this case would be radial and the two MMF sources wouldcomprise one (possibly solid) cylinder inside the smallest diameterrotor cylinder and one hollow cylinder outside the largest diameterrotor cylinder. Conceptually, this change in form begins by consideringthat one of the external MMF sources 62 becomes conical and grows itsmean diameter, the rotor discs 63 and stator discs 64 nest inside thisalso becoming cones and the other external MMF source.

[0185] All rotor discs 63 have the same number of magnetic poles, N_(R),and these are in the same angular orientation for each rotor disc.Similarly, all stator discs 64 have the same number of magnetic poles,N_(S), and these are in the same angular orientation for each statordisc and for the external MMF sources 62. In the above embodiments,N_(S)=2 and N_(R)=4. Any pair of pole numbers {N_(S),N_(R)} will producethe desired net lateral force provided that the following constraintsare observed

|N _(R) −N _(S)|=2 and N_(R) ·N _(S)≠0

[0186] In general, it will be attractive to use the lower pole numberfor the stator discs 64 for the purposes of minimising the frequency ofalternating magnetic flux in the stator discs and hence minimisinglosses. Higher pole numbers will tend to increase the proportion ofcopper which is active and reduce the axial depth needed in the tootheddiscs 65. However, they also increase the frequency of alternation ofmagnetic flux for a given shaft speed and they also demand higher updaterates in the active controllers.

[0187] It is required that the stator discs 64 should be able to passalternating magnetic flux in the axial direction without substantiallosses. In the above specific embodiments, it was stated that the statordiscs could be constructed as rolls of steel lamination. Otherconstruction methods are also possible including use of a powdermetallurgy composite having high resistivity and the use of a compositematerial in comprising a large fraction of axially-oriented magneticwire.

[0188]FIG. 41 should not be construed to imply that the internaldiameter of the rotor is necessarily small. In fact, there is nolimitation on the internal diameter. Large internal diameters will workwell.

[0189] Specific Embodiment “G”. A Linear Magnetic Bearing.

[0190]FIG. 46 illustrates a linear bearing in which a carriage 70comprising a first bearing member 71 of material of low magneticpermeability carries a set of spaced rectangular plate elements 72 eachformed of a plurality of alternating strips of ferro-magnetic andnon-ferro-magnetic material are interleaved with spaced elongate ribbonelements 75, again formed of a plurality of alternating strips offerro-magnetic and non-ferro-magnetic material. The spaced elongateribbon elements 75 are carried by a second bearing member 74 of materialof low magnetic permeability, and the ribbon elements 75 and the secondbearing member 74 constitute a rail which supports the carriage 70.

[0191] A permanent magnet 73 forming part of the carriage 70 gives riseto magnetic flux lines 76 which cross the gaps between the interleavedelements 72, 75 in zig-zag manner and gives rise to magnetic shearstresses generating forces which support the carriage over the rail.Side to side bearing forces may be generated in another way.

[0192] It will be noted that in this embodiment, it is convenient tomount the magnet 73, the source of MMF, not on the stator, the rail 74,75, but rather on the “rotor” i.e. the carriage 70. In general, in thecase of linear bearings, it will be convenient to mount the main sourceof MMF on the shorter of the two bearing members.

1. A magnetic bearing wherein each of two bearing members carries a setof bearing elements and the bearing elements of a said set carried byone member are interleaved with the bearing elements of a said setcarried by the other member to define three or more substantiallyparallel interleaf gaps between successive elements so that bearingforces can be developed as a result of magnetic shear stresses actingacross those gaps.
 2. A magnetic bearing according to claim 1, whereinat least one source of magneto-motive force is in place such that atleast one set of flux lines comes to exist which crosses three or moreof the interleaf gaps and wherein at least the majority of the bearingforce is developed as a result of magnetic shear stresses acting acrosssuch interleaf gaps.
 3. A magnetic bearing according to claim 1, whereinthe source(s) of magneto-motive force is or are arranged so that asingle set of flux lines comes to exist which crosses a set of three ormore of the interleaf gaps and wherein at least the majority of thebearing force is developed as a result of magnetic shear stresses actingacross that set of interleaf gaps.
 4. A magnetic bearing force accordingto claim 3, wherein substantially all the interleaf gaps are containedwithin said set.
 5. A magnetic bearing according to any preceding claim,wherein electrically conductive material is arranged within one or moreof the interleaved bearing elements to allow the flow of electriccurrents in order to influence the path of magnetic flux across at leastone interleaf gap.
 6. A magnetic bearing according to any precedingclaim, wherein permanent magnet material is distributed within at leastone of the interleaved bearing elements in order to influence the pathof magnetic flux across at least one interleaf gap.
 7. A magneticbearing according to any preceding claim, wherein ferro-magneticmaterial is distributed pattern-wise within at least one of theinterleaved bearing elements such that the reluctance experienced by aline of magnetic flux passing from one side of the bearing element(s) tothe other is a strong function of the location of that flux line; thisdependence of reluctance on location then serving to influence the pathof magnetic flux across at least one interleaf gap.
 8. A magneticbearing according to any preceding claim, wherein the bearing is anactive bearing.
 9. A magnetic bearing according to any of claims 1 to 7,wherein the bearing is a passive bearing.
 10. A magnetic bearingaccording to any one of the preceding claims, wherein the bearing isconstituted as a rotational bearing.
 11. A magnetic bearing according toany one of claims 1 to 10, wherein the bearing is constituted as alinear bearing.
 12. A magnetic bearing according to claim 10, whereinthe interleaving elements of the first and second bearing members areannular discs mounted normal to the axis of rotation.
 13. A magneticbearing according to claim 10, wherein the interleaving elements of thefirst and second bearing members are cylinders mounted coaxially withthe axis of bearing rotation.
 14. A magnetic bearing according to anyone of the preceding claims, wherein one said bearing member has onemore interleaving element than the other.
 15. A magnetic bearingaccording to any one of the preceding claims and substantially as hereindescribed.
 16. A magnetic bearing substantially as herein described withreference to any of FIGS. 9 to 47 of the accompanying drawings.